Calcium testing in marine systems is fraught with difficulties. Test kits generally don't work well in the ranges run in reefs, and many ions in marine water interfere with the kit. If your calcium level does not make sense, try diluting the aquarium water sample 50/50 with distilled, deionized or R/O water and testing. Multiply the result by 2. This will both save on valuable reagents in your kit and generally give a more accurate reading.
(You may want to consult with the kit manufacturer to determine that doing this will give valid results with your brand of kit).
If you have trouble maintaining calcium or buffer level, check to see that your salt mix has adequate magnesium. To save money and increase dissolvability, some manufacturers of popular newer salts have included only half the level of magnesium necessary in their sea salt mixes, resulting in low calcium solubility and poor buffering!